Amazon has 6 different international stores:
Prices of books differ from country to country and also exchange rates fluctuate heavily lately. For example the euro dollar exchange rate has been changing heavily, pounds inclusive.
So depending on how different prices in different countries are set and the current exchange rate you can save money by buying the book you want to read in another Amazon shop from where you normally buy. What you can do is search for price of the book you want to buy on each of these sites add the shipping costs and do the exchange rate conversion. This can easily save you 30-60% on your purchase.
There are thousands of books at amazon.co.uk that you can buy at cheaper rates than your regular book stores in Nigeria
You can shop from amazon.co.uk and get your books within 2 weeks from day of payment